Focussing device for electronic tubes

ABSTRACT

A device for focussing an electron beam in an electronic tube, using an axial magnetic field. It is constituted by a stack of permanent magnets, disposed in an annular fashion around the electronic tube; said magnets consist of washers, whose magnetisation is normal to their faces.

United States Patent Blamoutier et al.

FOCUSSING DEVICE FOR ELECTRONIC TUBES inventors: Michel Blamoutier; Jean Fraleux, both of Paris, France Assignee: Thomson-CSF, Paris, France Filed: Jan. 22, 1974 Appl. No.: 435,483

Foreign Application Priority Data Jan. 26, 1973 France 73.02837 US. Cl. 335/210, 3l5/5.35 Int. Cl. H0lf 7/00 Field of Search 335/210, 303, 306;

[ 1 Jan. 7, 1975 [5 6] References Cited UNITED STATES PATENTS 3,207,961 9/1965 Lohr 335/274 X 3,329,915 7/1967 Meyerer 335/210 3,781,592 12/1973 Harrold 335/210 X Primary ExaminerG. Harris Attorney, Agent, or FirmRoland Plottel, Esq.

[5 7] ABSTRACT A device for focussing an electron beam in an electronic tube, using an axial magnetic field. it is constituted by a stack of permanent magnets, disposed in an annular fashion around the electronic tube; said magnets consist of washers, whose magnetisation is normal to their faces.

5 Claims, 7 Drawing Figures WENTEU v 8.859.613

saw 1 BF, 3

me 1 jv FOCUSSING DEVICE FOR ELECTRONIC TUBES The present invention relates to the field of focussing an electron beam in an electronic tube.

In an electronic tube, it is well known to provide a device the function of which is to prevent divergence of the electron beam, this device being known as a focussing device. In certain applications, a particularly strict concentration of a beam on a target is needed, for example in electronic tubes intended for camera applications in television works. As a matter of fact, in tubes of this kind, the target is constituted by a photosensitive surface on to which the image which is to be transmitted is projected; the tube comprises means for producing an electron beam and means for creating an electric or magnetic deflecting field, which makes the beam scan said photosensitive surface, so that video signals are produced. In order to improve the transmission of the image and to improve the efficiency of the device as well, it is necessary that the beam, at impact on the photosensitive target, should be as concentrated as possible.

The known focussing devices generally employ an axial magnetic field, created by magnetic coils arranged around the tube; this solution has the drawback of consuming power, being bulky and being heavy, and more, in certain cases, introduces a limitation in the quality of the transmitted image, this being created by the heating of coils and target.

Also, known are focussing devices, constituted by a permanent magnet which enables said latter limitation to be avoided. They are constituted, for example, by a magnet generally of cylindrical form and radially magnetised, arranged around the electronic tube, the lines of force of the field, over some of their length, being orientated in the direction of the tube axis. However, in devices of this kind, only half their length can generally be utilised effectively, to create the focussing field, because of the configuration of the magnetic field in these devices; the consequence, therefore, is that these devices are excessively bulky.

In accordance with the present invention, there is provided a focussing device for an electronic tube, through which an electron beam is propagating, said device comprising a permanent magnet surrounding said tube in an annular fashion, in the direction of the propagation of said electron beam, said magnet being constituted by a coaxial stack of rings magnetised in a direction substantially normal to their faces.

The invention will be better understood from a consideration of the ensuing description and the related drawings, in which:

FIG. 1 illustrates a sectional view of an embodiment of an elementary annular magnet in accordance with the invention;

FIG. 2 illustrates the variation of the magnetic field produced by the magnet described in FIG. 1;

FIG. 3 illustrates another embodiment of an elementary annular magnet in accordance with the invention;

FIGS. 4 and 5 are similar to FIGS. 1 and 2, but relate to the embodiment shown in FIG. 3;

FIG. 6 illustrates a preferred embodiment of the f0- cussing device in accordance with the invention;

FIG. 7 illustrates the variation in the magnetic field, produced by the device shown in FIG. 6.

FIG. 1 illustrates a sectional view of an elementary annular magnet (l), in accordance with the invention,

as well as the magnetic lines of force (1 l, 21) produced by that magnet.

In accordance with this embodiment, the magnet l is constituted by a flat, circular ring of rectangular section, 10, with an external diameter D and internal diameter D,- and an axis of revolution XX substantially coincidental with the axis of propagation of the electron beam through the electronic tube. The magnet is made of a magnetic material having a magnetisation normal to its faces, that is to say along the axis XX, normal to the plane of the ring, this plane having been marked in the figure by the axis Y'Y. In a preferred embodiment, said magnetic material is constituted by a plastic material in which ferrite powder has been incoporated. An elastomer could equally well be used.

A magnet of the kind indicated by the reference 1, produces, as those skilled in the art will be aware, a magnetic field whose shape corresponds with those which would be obtained, on the one hand, from a solid disc of diameter D, uniformly magnetised over the whole ofits surface, and on the other hand, from a solid disc of diameter D,- likewise uniformly magnetised over the whole of its surface but in the opposite direction to the preceding disc. In the figure, half the lines of force corresponding to this kind of configuration, have been shown, the other half of these lines deriving from the first by a simple symmetry in relation to the axis XX. Thus, two sets of lines of force are obtained, respectively 11 and 21, which produce a change in the direction of the field along the axis XX, and the points A and B, which will be referred to as singular points. The position of these singular points, symmetrical in relation to the axis Y'Y, is a function of the ratio of the diameters D,-/D They are, in other words, the further away from one another the larger the internal diameter 0,.

FIG. 2 illustrates the variation (graph 20) in the strength of the magnetic field along the axis XX. The graph 20 shows that the axial component H, of the magnetic field is symmetrical in relation to the axis Y'Y, have a maximum at the centre of the magnet l, on the axis Y'Y, a zero at the singular points A and B, and its sign changes after these points. The strength of the field at the centre of the magnet depends, of course upon the diameter D,-: it is the higher the smaller said diameter D,-.

To manufacture the focussing device in accordance with the invention, preferentially a stack of elementary annular magnets having the same axis XX will be used, the magnets having the same dimensions and all having the same magnetisation level, the electronic tube being arranged upon said axis, inside the stack. Contact takes place between faces of opposite magnetism. The graph illustrating the variation of the axial component H, of the magnetic field created by this kind of stack arrangement, is obtained from graphs corresponding to each of the magnets;,its general shape, although not shown, is similar to that illustrated in FIG. 2, and has two singular points.

In a stack of this kind, the singular points of each magnet must be sufficiently far away from one another, for the component H, of the field created by one elementary magnet, not to be substracted from the component created by the following elementary magnets, and this leads to the choice of a large diameter D,-; furthermore, the larger this diameter is, the lower the level of the ordinate positions of the points located below the abscisse axis of FIG. 2. In addition, said diameter D,- must not be too large, in order that the component H, shall be of sufficient magnitude in accordance with what has been said earlier. It has been found that a ratio D,-/D at least equal to 0.6, satisfies these requirements.

FIG. 3 illustrates another embodiment of an elementary magnet in accordance with the invention. It is constituted by a ring 30 with chamfered edges and an axis of revolution XX, the cross section 31 of which is rectangular and the faces 32 and 33 of which are not flat surfaces but frustoconical surfaces of axis XX. It can be made of the same magnetic as the magnet 1.

FIG. 4 is a sectional view of a magnet 30 showing the lines of force produced by this kind of magnet when magnetised in a direction normal to its face 32 and 33.

The configuration of the magnetic field is produced in the same way as before (FIG. 1), but is no longer symmetrical in relation to the plane passing through the center of the cross section of the ring, the line of which in the plane of the figure is the axis YY, the section 31 itself no longer being symmetrical in relation to said plane. Thus, two sets of lines 41 and 51 are obtained and two singular points C and D which are not symmetrical in relation to the axis YY, the point C being the furthest from said axis.

FIG. 5 illustrates the variation of the magnetic field component H, along XX.

This component, as before, is at a maximum on the axis YY, zero at the singular points C and D and adopts opposite sign after said points, but is no longer symmetrical in relation to the axis YY.

FIG. 6 illustrates the focussing device in accordance with the invention, in a preferred embodiment in which it is constituted by two half stacks of elementary magnets as described in FIG. 3, the conditions being the same as those specified in relation to the earlier stack.

Each half stack is designed, on one and the same axis XX, so that the apices of the cones to which the faces 32 and 33 belong, are located at the same side of the half stack; the two half stacks are disposed coaxially, face to face, in contact with one another through their larger bases in the example of the figure.

In the figure, the two terminal magnets of each half stack have been shown: magnets 61 and 62 for the half stack whose apices are located to the left of the figure and magnets 71 and 72 for the other half stack.

FIG. 7 shows the graph of variation in the component H, along XX, of the magnetic field produced by the device described in FIG. 6.

Said graph is the sum of the graphs corresponding to each of the magnets; it is symmetrical in relation to the plane separating the two half stacks (plane of symmetry of the device), and it has two singular points E and F.

This graph is similar to the graph corresponding to a stack of magnets such as described in FIG. 1, but it has the advantage, over the latter, of having singular points (E and F) which are further away from one another, this by the utilisation of the asymmetry shown in FIGS. 4 and 5 in respect of each magnet: in other words, the terminal magnets 61 and 71 are arranged in such a fashion that it is their sinular point C located furthest away from their axis YY, which is located outside the stack.

The focussing device in accordance with the invention thus makes it possible in relation to devices which employ magnetic coils, to avoid heating up and power consumption and thus to effect a limitation far as the operating voltage of the electronic tube is concerned.

The device in accordance with the invention furthermore has the advantage of a reduced bulk, all the length of the device effectively participating in the formation of the focussing magnetic field.

In addition, the material used, which is an electrical insulator having a certain degree of elasticity, makes it possible on the one hand to avoid signal damping and disturbances due to Foucaults currents, and on the other hand to give the electronic tube around which the magnets are arranged, a certain degree of protection against mechanical shock. Finally, the very simple structure of this device, makes it possible to achieve a substantial reduction in manufacturing costs.

This focussing device, applied to electronic tubes for television cameras, has made it possible to transmit an image made up of 900 points over a line of 12.6 mm.

The breaking down of the device in accordance with the invention into elementary ring-shaped magnets, as described hereinbefore, makes it possible to create other variant embodiments, adapted to particular applications, than those of the aforesaid examples, in which rings of different dimensions and different magnetisation levels are used. All these variants fall within the scope of the invention, which is not limited to the embodiment described and shown which was given solely by way of example.

What is claimed is:

l. A focusing device for an electronic tube, through which when the device is in use, an electron beam is propagating, said device comprising a permanent magnet surrounding said tube in an annular fashion, in the direction of the propagation of said electron beam, said magnet being constituted by two coaxial half-stacks of rings, each of said rings being magnetized in a direction substantially normal to their faces, and being a thin, rectangular-section, frustoconical ring, each half-stack, being made up of a certain number of said rings, assembled together and in contact with each other through faces of opposite magnetic type, the two half-stacks being located opposite one another with their larger bases, their mutually opposite faces having opposite magnetic types.

2. A focussing device as claimed in claim 1, wherein the internal diameter of each of said rings is at least equal to six-tenths of the external diameter thereof.

3. A focussing device as claimed in claim I, wherein all the rings are identical and have the same magnetisation level.

4. A focussing device as claimed in claim 1, wherein said rings are made of a plastic material filled with ferrite powder.

5. A focussing device as claimed in claim 1, wherein der filler. 

1. A focusing device for an electronic tube, through which when the device is in use, an electron beam is propagating, said device comprising a permanent magnet surrounding said tube in an annular fashion, in the direction of the propagation of said electron beam, said magnet being constituted by two coaxial halfstacks of rings, each of said rings being magnetized in a direction substantially normal to their faces, and being a thin, rectangular-section, frustoconical ring, each half-stack, being made up of a certain number of said rings, assembled together and in contact with each other through faces of opposite magnetic type, the two half-stacks being located opposite one another with their larger bases, their mutually opposite faces having opposite magnetic types.
 2. A focussing device as claimed in claim 1, wherein the internal diameter of each of said rings is at least equal to six-tenths of the external diameter thereof.
 3. A focussing device as claimed in claim 1, wherein all the rings are identical and have the same magnetisation level.
 4. A focussing device as claimed in claim 1, wherein said rings are made of a plastic material filled with ferrite powder.
 5. A focussing device as claimed in claim 1, wherein said rings are made of an elastomer with a ferrite powder filler. 